An aqueous fluoropolymer dispersion synthesized by emulsion polymerization (hereinafter also referred to as a fluoropolymer dispersion) is generally unstable and, when subjected to a mechanical shearing force or the like, produces a coagulation that cannot be redispersed. Accordingly, such dispersions are stored and sold in a concentrated and stabilized form obtained by adding a polyoxyalkyl ether or other such nonionic surfactant, an example of which is Triton X-100 (trademark; hereinafter also referred to as TX-100) made by Union Carbide. An aqueous fluoropolymer dispersion has a higher specific gravity than an aqueous hydrocarbon dispersion. For instance, an aqueous dispersion of polytetrafluoroethylene has a tendency of fibrillate, and requires a relatively large quantity of surfactant (5 to 20 wt %). Applications of such fluoropolymer dispersions include various coating agents, electrode binders, and electrode water repellents, as described in Japanese Unexamined Patent Publications S45-39829, H2-158055, and H2-291665, and elsewhere.
However, when a fluoropolymer dispersion containing a nonionic surfactant such as Triton X-100 is used as an electrode binder, which is an application obtained by way of coating and drying steps, the surfactant remains in the electrode coating film, and it is undesirable for the surfactant to remain in a cell system because the residual surfactant will lead to an increase in internal resistance, deterioration of cycle characteristics, and so forth.
On the other hand, in terms of the stability of a fluoropolymer dispersion, the addition of a surfactant is indispensable, so it can be said that it is the most desirable to use a surfactant that can be completely removed from the inside of the coating film by pyrolysis, evaporation, volatilization, or the like, in an electrode coating film drying step during electrode production.
However, up to now, the aspects of compatibility with the fluoropolymer, adhesion, foaming resistance, stability against a shearing force, and so forth, have been emphasized in the selection of a surfactant, and there was no fluoropolymer dispersion that made use of a surfactant as described above.
It is an object of the present invention to provide an electrode additive suitable for the production of an electrode with no or substantially no residual substance originating in a surfactant.
It is also an object of the present invention to provide a stable aqueous dispersion composition containing a fluoropolymer.
It is a further object of the present invention to provide a method for manufacturing an electrode with no or substantially no residual substance originating from a surfactant.